Magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities. As the grain size of the magnetic recording medium is decreased in order to increase the areal density, a threshold known as the superparamagnetic limit at which stable data storage is no longer feasible is reached for a given material and temperature.
Thermal stability of magnetic recording systems can be improved by employing a recording medium formed of a material with a very high magnetic anisotropy. However, very few of such hard magnetic materials exist. Furthermore, with currently available magnetic materials, recording heads are not able to provide a sufficient magnetic writing field to write on such materials.
The current strategy to control media noise for high areal density recording is to reduce the lateral dimensions of the grains. The resulting reduction of the grain volume has to be compensated by a corresponding increase of the magnetic crystalline anisotropy energy density of the media in order to ensure thermal stability of the stored bits throughout a period of at least 10 years. Although the high magnetic crystalline anisotropy of recently developed granular media like L10 based FePt or CoPt supports areal densities up to several Tbit/inch2, it also hinders conventional writing.
One solution to overcome this dilemma is to soften the medium temporarily by locally heating it to temperatures at which the external write field can reverse the magnetization. This concept, known as heat assisted magnetic recording (HAMR), relies on proper management of the spatial and temporal variations of the heat profile. HAMR involves locally heating a magnetic recording medium to reduce the coercivity of the recording medium in a confined region so that the applied magnetic writing field can more easily direct the magnetization of the recording medium in the heated region during the temporary magnetic softening of the recording medium caused by the heat source. HAMR allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature assuring a sufficient thermal stability.
Lateral heat diffusion in HAMR media is a factor which must be considered when establishing the desired dimensions of the region to be heated. Typical dimensions for Tbit/inch2 recording are 25×25 nm2, assuming a bit-aspect-ratio of one. If the heat delivery system delivers an intensity profile with Gaussian FWHM of 25 nm, then no additional heat spread in the media can be tolerated.
Width and curvature of transitions written via HAMR are governed by the shape of the lateral heat profile. While the transition curvature follows the isothermal line for the chosen write temperature, the transition width is proportional to the temperature gradient in the track direction. Optimal signal-to-noise ratios of the read back signal are obtained for transitions that are straight in the cross track direction. Maximum areal densities are obtained for minimum transition widths that are constant across the track. Hence, rectangular temperature profiles are superior to circular profiles.
Temperature gradients are the driving force for heat diffusion, which in turn leads to reduced temperature gradients. The speed of heat diffusion in a given medium is governed by the thermal diffusivity of the medium. For media with isotropic thermal diffusivity, any initially rectangular temperature profile will quickly be transformed into a circular profile. In contrast, vertical thermal diffusion from a uniformly heated thin film into a patterned heat sink comprising regions of low thermal diffusivity embedded in a matrix of high thermal diffusivity material will lead to local variations of the film temperature that resemble the heat sink pattern.
Other important aspects of HAMR are the efficiency of the heat delivery system and the cooling rate of the media. While the heating has to be sufficient to heat the media to temperatures approximating the Curie point of the media, the cooling rate has to be fast enough to avoid thermal destabilization of the written information during the time the media cools down. Efficiency of the heat delivery system and fast cooling rate are competing factors. Faster cooling rates require more heating power for a certain temperature increase.
A need exists for recording films that can effectively control heat transfer for heat-assisted magnetic recording and other types of systems.